Create propagation_p368.py

[fgiovanard]

[bwinkel]

Dear @fgiovanard,

thanks a lot for your first Pull Request! There a a few minor things, which we'll need to fix before merging; see comments.

A general remark, please make a few changes to the formatting. I like to stick to pep8 as much as possible. If you want a quick fix, simply use black to reformat it automatically. I think, you could also simply use this online tool for reformatting.

Let me know if you need help.

[bwinkel]

I forgot to say, please also add a test into the tests subdirectory. (There are plenty of examples about how it's done.)